Add nightly schedule for UI Tests

[DonJayamanne]

For #4692

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• [n/a] Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Appropriate comments and documentation strings in the code
• [n/a] Has sufficient logging.
• [n/a] Has telemetry for enhancements.
• [n/a] Unit tests & system/integration tests are added/updated
• [n/a] Test plan is updated as appropriate
• [n/a] package-lock.json has been regenerated by running npm install (if dependencies have changed)
• [n/a] The wiki is updated with any design decisions/details.